Device for increasing the functional reliability of a hydraulic circuit of a hydraulically operable clutch

ABSTRACT

A known device for increasing the functional reliability for a hydraulic circuit of a hydraulic clutch having one pressure reservoir and one release cylinder which comprises a primary piston, a secondary piston and a compensation volume is to be further developed in the sense that when the vehicle is at a standstill and the parking brake released, an unintentional release of the clutch is prevented.  
     According to the invention there is used for the purpose a pressure-monitoring/pressure-supply device ( 28 ) which determines a reference operating pressure and establishes a static pressure which in case of system errors allows only a displacement of the secondary piston ( 16 ) such that the contact pressure of the clutch discs against each other is still sufficient to hold the vehicle at a standstill.

According to the preambles of claim 1 and of claim 7, the inventionconcerns a device for increasing the functional reliability for ahydraulic circuit of a hydraulically operable clutch.

An increase of the functional reliability of the hydraulic circuit of ahydraulically operable clutch generally aims at maintaining the existingshifting state of the clutch and obtaining between the shifting states adefined transition characteristic which can be reproduced and thuseffectively controlled by the driver of the vehicle concerned.

To meet the requirements, pressure intensifiers are known from the priorart from which a sufficient feedback goes to the clutch pedal so thatthe engagement and disengagement ratios sought by the driver can beadapted to the existing situation. It is very important here toimplement play and wear compensation in order to ensure a coincidentresponse characteristic during the service life of the clutch.

Release cylinders thus are known from the prior art in which automaticwear compensation is implemented. In that case, the release cylinder canhave two pistons; the axial spacing between which is determined by thevolume of a liquid located between the pistons; there resulting anadaptation of the volume depending on the wear state of the clutch.

However, when the vehicle is stopped, there is the danger that in casethe working valve is not sealed, hydraulic liquid flows into theintermediate space between primary and secondary pistons, via a bleedinghole and, due to the volume increase, moves the secondary piston so thata slow opening of the clutch cannot be ruled out with certainty wherebythe vehicle, when the parking brake is open, could unintentionally setitself in motion.

In DE 197 17 486 C2 is described a pneumatic pressure intensifier for ahydraulic clutch operation for motor vehicles. In the pressureintensifier, a pneumatic working space is provided which can be loadedwith pressurized air for pressure intensification of a hydraulic workingspace. One control piston, coordinated with the pneumatic working spaceand serving for engaging and disengaging the pressure intensificationsource, is activated by a working piston located in the hydrauliccircuit so that a leakage, due to pressure increase in the releasecylinder, does not occur.

However, the described pneumatic intensifier of the device has acomplicated construction and, to that extent, is susceptible to noiseand comparatively costly to produce.

Therefore, the invention is based on the problem of increasing thefunctional reliability of an operation in the hydraulic circuit ofhydraulic clutch by comparatively simple means.

According to the invention, in a device as defined in the preamble ofclaim 1, with the pressure reservoir is coordinated onepressure-monitoring/pressure-supply device which determines a referenceoperating pressure and a differential pressure so as to reduce theoperating pressure when the vehicle is stationary and the engine isdisconnected for obtaining a residual static pressure.

The invention is associated with the advantage that when the vehicle isstarted again after stoppage with disconnected motor, as a result of thesteadily provided static pressure the hydraulic clutch, is quicklyavailable and, at the same time, the stress of the pressure reservoir isreduced since it is not exposed to the full operating pressure. Thepermanently abutting full operating pressure can lead to an intensifieddiffusion of the operating medium, particularly when the pressurereservoir is designed as a diaphragm reservoir. The consequence would beincreased operating and maintenance costs. It is further ensured thatwhen static pressure abuts, the clutch does not open or can open only tothe extent that a sufficiently high drag torque is retained on theclutch so that the vehicle does not roll away.

In a preferred development of the invention, the inventive device hasone pressure-limiting unit which makes an adjustment and limitation ofthe static pressure possible based on a maximum value which, when actionis exerted upon the liquid volume located between primary and secondarypistons, allows only a displacement of the secondary piston of therelease cylinder such that the contact pressure of the clutch pressureplate on the clutch disc still suffices to keep the vehicle at astandstill. It is thus ensured that when a gear is introduced and theparking brake released, even on an inclined road, the vehicle is held inthe position concerned.

To determine the maximum admissible static pressure, it is possible touse the relative displacement of the piston rod that actuates therelease lever of the clutch. The piston rod can thus remain in operativeconnection with an incremental travel measuring system from which theinformation, concerning the covered path of the piston rod, can berelayed as standard to the pressure-monitoring/pressure-supply devicefor establishing the functional dependence of the static pressure on theaxial displacement of the piston rod.

In addition, it is basically possible to detect the distortioncharacteristics of the piston spring and set it in relation to themomentarily prevalent pressure.

To prevent the selected static pressure from lowering, due to innerleakages in the hydraulic circuit which would result in delaying theavailability of the vehicle after a period of time without operation, ina convenient development of the invention, one trigger element iscoordinated with the pressure-monitoring/pressure-supply device which,preferably timed, causes a revision of the static pressure. The staticpressure is conveniently corrected when a reference pressure, deemedadmissible, is fallen below. For the triggering, the appertainingelectronic system always has to be activated only briefly, e.g., onceper hour which as a consequence, will have only an unimportantconsumption of energy. Should the pressure fall below the preset valuedetermined, then the pump, or any other such device for producing theoperating pressure, has to be operated until the required operatingpressure is again produced. For the driver, corresponding informationcan be provided in case the leakage rate in the hydraulic system shows asignificant error in the system which can be detected when, in thecourse of time, the pressure drop exceeds a specific value.

Should the possibility be ruled out that under action of the staticpressure upon the release cylinder there generates a volume increasebetween primary and secondary pistons and thus a force action upon therelease lever of the clutch which leads to a reduction of thetransmissible torque and to a partial opening of the clutch then, indevelopment of the invention, by way of control of a working valve, theprimary piston can be displaced to the extent of shutting a bleedinghole communicating with the compensation volume. Due to the volumeincrease in the intermediate space, there also exists the danger thatwhen a clutch is actuated, the diaphragm spring between the two pistonscould be over-pressed and destroyed and that the clutch no longer closesagain or closes with great delay.

When the clutch is closed, the primary piston of the release cylinder ispositioned so that the bleeding hole of the primary piston is shut inorder that no change of volume affects the liquid volume located betweenprimary and secondary pistons.

To allow a wear compensation of the piston, the pressure control unit isbriefly deactivated by a trigger switch in a preferred development ofthe invention so that the primary piston of the release cylinder, for avolume compensation, releases the bleeding hole to the space between theprimary and the secondary pistons. The triggering ensures that anopening of the bleeding hole has not to occur upon each actuation of theclutch, but only on and off, and yet a sufficient wear compensationresults and the above described danger is prevented. A triggered travelrecording of the piston can ensure that the vehicle does not roll away.An adjustment of the clutch would be detected and the static pressureadapted.

The invention is explained in detail herebelow with reference to apreferred embodiment and appertaining drawings. The drawings show:

FIG. 1 is the basic construction of a hydraulic clutch actuation deicewith a pneumatic intensifier;

FIG. 2 is a release cylinder of a clutch actuation according to FIG. 1with primary and secondary pistons; and

FIG. 3 is a graph of the release cylinder with symbolic tying of apressure-monitoring/pressure-supply device and one pressure controlunit.

A hydraulic clutch actuation device with pneumatic intensifieressentially consists, according to FIG. 1, of a master cylinder 3provided with a compensation tank 1 and actuated by a clutch pedal 2,one pressure intensifier 4 rear-mounted on the master cylinder, onerelease cylinder 5 acting upon the release mechanism of the clutch andhydraulic pipes 6 and 7 which create an impermeable connection betweenthe master cylinder 3 and the pressure intensifier 4, the same asbetween the pressure intensifier 4 and the release cylinder 5.

The pressure intensifier 4 has one pneumatic area 9 which acts uponhydraulic area 8 and the adjoining hydraulic pipes 6, 7 and in an activestate is loaded with a compressed gas provided by a compressor 10 via apneumatic line 11.

In FIG. 2, the release cylinder 5 is not shown in a controlled state. Ahousing 12, basically designed of cylindrical shape, is provided with astepped hole 13 one primary piston 14 with a seal ring 15 and onesecondary piston 16 with a seal ring 17 are axially movably disposed. Aspacing between the primary piston 14 and the secondary piston 16 andthus the practical possibility of creating a free space for an automaticwear compensation of parts of the clutch is achieved by a spiral spring18 which limits the approximation of the secondary piston 16 to theprimary piston 14 in case of pressure relief of the primary piston 14.The secondary piston 16 acts upon a piston rod 19 which is passed into arecess of a front plate 20 of a release cylinder 5 and extends to therelease lever 21 of the clutch actuation. For protection, the piston rod19 is surrounded by bellows which by their terminal area sit upon thefront plate 20 of the housing 12.

On the end of the housing 12 remote from the piston rod 19, thehydraulic pipe 7 is impermeably connected with a connecting area 22 ofthe housing 12. One bleeding hole 23 provided in the connecting area 22produces in the terminal side stop of the primary piston 14 a connectionof the hydraulic pipe 7 with a compensation space 26 between the primarypiston 14 and the secondary piston 16 so that the liquid in thecompensation space 26 between the primary piston 14 and the secondarypiston 16 communicates with the liquid provided via the hydraulic pipe7. The space between the piston 14 and 16 can be altered by altering theliquid volume enclosed between the primary piston 15 and the secondarypiston 16, thus achieving a wear compensation in the area of the clutch.

As shown in FIG. 3, the release cylinder 5 of the hydraulic clutchactuation is controlled by a working valve 24 designed as 2/2directional seat valve which, in one of its shifting states, relays thehydraulic system pressure originating from a pressure reservoir 25 tothe connecting area 22 of the release cylinder 5 after the primarypiston 14 is axially displaced from its end position and shuts thebleeding hole 23. By way of the liquid of constant volume in thecompensation space 26 between primary piston 14 and second piston 16there result the movement of the primary piston 14, the same as of thesecondary piston 16 and also the piston rod 19 whereby the release lever21 is actuated and the clutch opened.

One pump 34 driven by a motor 29 conveys hydraulic liquid from a tank27, via a recoil valve 35, in direction toward the pressure reservoir25. In order that the pressure is monitored in the system, apressure-limiting valve 36 is provided which allows hydraulic liquid toflow back to the tank 27 should it be necessary.

When the clutch closes, the spring-loaded release lever 21 moves back toits original position in FIG. 3 and, at the same time, produces an axialmovement to the left of the secondary piston 16 of the enclosedhydraulic liquid and primary piston 14 until the latter has reached itsfinal position in the housing 12 of the release cylinder. A workingvalve 33, designed as 2/2 directional seat valve, is in a shiftingposition here in which a volume return flow can result in the tank 27.Adapted shifting positions of the working valves 24 and 33 additionallyallow an oil volume compensation in an end position of the primarypiston 14, via the bleeding hole 23, released by the primary piston 14and thus an adaptation of the clutch actuation to the wearcharacteristics of the of the clutch.

If the vehicle is stopped and the motor is shut off, apressure-monitoring/pressure-supply device 28 receives a correspondinginformation on the basis of which the monitoring/pressure-supply device28 determining the system pressure and reducing it by a differentialpressure so that when gear has been introduced and parking brake isopen, the vehicle is prevented from rolling away even on inclined roadsand a quick availability of the clutch actuation is nevertheless ensuredfrom then on.

For this the pressure-monitoring/pressure-supply device 28 ensures thatthe pressure detected in the pressure-monitoring/pressure-supply device28 from system and differential pressure is limited to a static pressurewhich, during action upon the secondary piston 16 and thus upon pistonrod 19 and release lever 21, cannot lead to a partial release of theclutch. Such can always be the case when, due to error in the system,for example, when the working valve 24 does not exactly close, a slowpressure buildup occurs on the release cylinder 5 which, when thebleeding hole 23 is open, would result in a volume increase of theliquid-filled compensation space 26 between primary piston 14 andsecondary piston 16.

Therefore, by adequate minimal control of a shifting element, such asthe working valve 24, by way of a pressure control unit 30 provided inthe pressure-monitoring/pressure-supply device 28, due to the pressuregenerating on the release cylinder 5, the primary piston 14 is displacedin direction of the “clutch open” state and thereby the bleeding hole 23is kept closed while the clutch remains closed. Alternatively, theprimary piston 14 in the “clutch closed” state could always be movedforward to the extent that the bleeding hole 23 is shut. Then, onlyafter a clutch actuation, the primary piston is briefly moved back up tothe stop and the bleeding hole 23 opened in order to be subsequentlybrought again to the position in which the bleeding hole 23 is closed.Since the clutch wear per clutch operation is small, this methodsuffices to make a reliable wear compensation possible and shut thebleeding hole 23 when its operation is not needed. Thus an uncontrolledoil flow in the intermediate space between the primary and the secondarypistons 14, 16 is reliably prevented.

As one other alternative, the volume change between the primary piston14 and the secondary piston 16 and therewith a wear compensation of theclutch can also result by a trigger switch 31 which, likewise, can alsobe situated in the pressure-monitoring/pressure-supply device 28 and, atcertain intervals, briefly deactivates the shutting of the bleeding hole23 by the primary piston 14 to enable a volume exchange to take place.

Besides, the pressure-monitoring/pressure-supply device 28 communicateswith an incremental path sensor 32 by which the information relative tothe covered path of the piston rod 19 is relayed as standard to thepressure-monitoring/pressure-supply device 28 for establishing thefunctional dependence of the static pressure on the axial displacementof the piston rod 19.

The pressure control unit 30 can be designed so as to work independentlyof the pressure-monitoring/pressure-supply device 28, thus being activewhen the clutch is closed under different operation conditions and alsowhen the motor is active.

REFERENCE NUMERALS

-   -   1 compensation tank    -   2 clutch pedal    -   3 master cylinder    -   4 pressure intensifier    -   5 release cylinder    -   6 hydraulic pipe    -   7 hydraulic pipe    -   8 hydraulic area    -   9 pneumatic area    -   10 compressor    -   11 pneumatic pipe    -   12 housing    -   13 stepped hole    -   14 primary piston    -   15 seal ring    -   16 secondary piston    -   17 seal ring    -   18 spiral spring    -   19 piston rod    -   20 front plate    -   21 release lever    -   22 connecting area    -   23 bleeding hole    -   24 working valve    -   25 pressure reservoir    -   26 compensation space    -   27 tank    -   28 pressure-monitoring/pressure-supply device    -   29 motor    -   30 pressure control unit    -   31 trigger switch    -   32 path sensor    -   33 working valve    -   34 pump    -   35 recoil valve    -   36 pressure-limiting valve

1. Device for increasing the functional reliability for a hydrauliccircuit of a hydraulically actuatable clutch having one release cylinder(5) actuated by a master cylinder (3) via hydraulic pipes (6, 7) andwhich via one primary piston (14) and one secondary piston (16) withvariable spacing of said pistons (14, 16) proceeds to wear compensation,having one pressure reservoir (25) and one pressure-reservoir fillingdevice, characterized in that with the pressure reservoir (25) iscoordinated one pressure-monitoring/pressure-supply device (28) whichdetermines in the actuation system one reference operating pressure andone differential pressure in order that, when the vehicle is atstandstill and the motor shut off, the operating pressure be reduced toobtain a permanent static pressure.
 2. Device according to claim 1,characterized in that said device has one pressure-limiting unit (29)which makes possible an adjustment and limitation of the static pressurebased on a maximum value which during action upon the liquid volumelocated between the primary piston (14) and the secondary piston (16)allows only a displacement of the secondary piston (16) of the releasecylinder (5) such that the contact pressure of a plate of the clutchupon a clutch disc still suffices to prevent rolling away of the vehicleat standstill with introduced gear, released vehicle parking brake andinclined road.
 3. Device according to claim 2, characterized in thatindirect parameters not traced back to the static pressure prevalent inthe actuation system are relayed as standard to thepressure-monitoring/pressure-supply device (28) for establishing thefunctional dependence of the static pressure on the correspondingparameters.
 4. Device according to claim 1 or 3, characterized in thatwith the pressure-monitoring/pressure-supply device (28) is coordinatedone trigger element (31) which preferably timed produces a revision ofthe static pressure.
 5. Device according to at least one of thepreceding claims, characterized in that the primary piston (14) and thesecondary piston (16) are spaced apart in the release cylinder (5) by acompensation space (26) with variable volume.
 6. Device according to atleast one of the preceding claims, characterized in that by a control ofa working valve (24), the primary piston (14) is displaceable in therelease cylinder (5) to the extent of shutting a bleeding hole (23)communicating with the compensation space (26) in the release cylinder(5).
 7. Device for increasing the functional reliability for a hydrauliccircuit of a hydraulically actuatable clutch which has one releasecylinder (5) actuated by a master cylinder (3) via hydraulic pipes (6,7) and which via a primary piston (14) and a secondary piston (16) withvariable spacing of the pistons (14, 16) proceeds to wear compensation,having one pressure reservoir (25) and one pressure-reservoir fillingdevice, characterized in that with the pressure reservoir (25) arecoordinated one pressure-monitoring/pressure-supply device (28) and onepressure control unit (3) which acts upon a shifting element (24) sothat the primary piston (14) is slightly axially displaced to a positionin which a volume change of a compensation space (26) located betweenthe primary piston (14) and the secondary piston (16) is ruled out. 8.Device according to claim 7, characterized in that the pressure controlunit (30) at certain time intervals is briefly deactivatable by atrigger switch (31) so that a wear compensation is possible by way ofchanging the hydraulic volume enclosed in the compensation space (26)between the primary piston (14) and the secondary piston (16).